The invention relates to zone isolation tools for sealing portions of a well.
After a well has been drilled and the casing has been cemented in the well, one or more sections of the casing adjacent pay zones are perforated to allow fluid from the surrounding formation to flow into the well for production to the surface. Perforating guns are lowered into the well and the guns are fired to create openings in the casing and to extend perforations into the surrounding formation. In the well shown in FIG. 1, two perforated regions 14 and 16 in the formation are shown next to two different sections of the casing 12 in a well 10.
Contaminants (such as water or sand) are sometimes produced along with the oil and gas from the surrounding formation. In the system shown in FIG. 1, during production, fluid flows from the perforated regions 14 and 16 through perforated openings in the casing 12 into the bore 20 of the well 10. The fluid then rises up through a production tubing 18 to the surface. A packer 22 positioned near the bottom of the production tubing 18 is used to seal off well fluids from the annulus 24 between the production tubing 18 and the casing 12.
If contaminants are detected in the fluid from the production tubing 18, then a logging tool is lowered into the well 10 to determine the source of the contaminants. If, for example, the source of contaminants is the perforated region 14, then the perforated openings in the casing 12 are sealed to prevent fluid flow from the perforated region.
To seal the desired section of the casing 12, one technique typically used is referred to in the industry as a "squeeze job." First, the production tubing 18 is removed from the well. Then, the zone in the casing 12 adjacent the general area of the perforated region 14 is isolated using temporary packers. Cement is pumped down the bore 20 through a tube to the isolated zone to seal the perforated openings in the desired section of the casing 12. Drilling out of the cement is then required if production is desired from a lower payzone.
Another technique has been proposed for sealing casing sections downhole, which is described in J. L. Saltel et al., "In-Situ Polymerization of an Inflatable Sleeve to Reline Damaged Tubing and Shut-Off Perforations," Offshore Technology Conference, pp. 1-11 (May 1996). A cable carrying seven electrical conductors is used to lower an inflatable sleeve which carries a permanent sleeve (comprised of resins, fibers, and elastomers) downhole. The inflatable sleeve is pressurized to push the permanent seal against the inside surface of the casing. Electric power provided down the wireline from the surface is used to generate heat to increase the temperature of the resin for a sufficient period of time to cross link (or "cure") the resin in the permanent sleeve. The permanent sleeve is left downhole to maintain a seal over perforated sections of the casing.
The electrical energy required to cross link the resin in the system of Saltel et al. varies between 400 W/m and 1,900 W/m, depending upon the diameters of the casing. To provide the necessary electrical energy, a 1,250-volt DC supply is used at the surface to generate greater than about 2.5 amps of current through each of the seven conductors and the associated resistive elements.